1. Field of the Invention
The present invention relates generally to an air-fuel ratio control system for an internal combustion engine, and more specifically, to the air-fuel ratio control system, wherein an air-fuel ratio feedback control is performed based on an output from a sensor which is provided on the upstream side of a catalytic converter in an exhaust passage for monitoring the exhaust gas passing therethrough to detect an air-fuel ratio of an air-fuel mixture which has caused the monitored exhaust gas.
Hereinafter, for simplification of explanation, the expression "air-fuel ratio" will be used to represent not only "an air-fuel ratio of an air-fuel mixture to be fed to the engine", but also other meanings where the context allows. For example, the expression "air-fuel ratio" will also represent "an air-fuel ratio indicative or related condition of the monitored exhaust gas" or "a converted value of an air-fuel ratio", depending on the context.
2. Description of the Prior Art
Japanese First (unexamined) Patent Publication No. 2-238147 discloses an air-fuel ratio control system for an internal combustion engine of the above-noted type.
In the system of this publication, oxygen concentration sensors (hereinafter referred to as "O.sub.2 sensors") are respectively arranged on the upstream and downstream sides of a catalytic converter. When it is determined based on an output voltage of the upstream O.sub.2 sensor that an air-fuel ratio of the exhaust gas is deviated or fluctuated to a rich or lean side with respect to a stoichiometric air-fuel ratio, an air-fuel ratio correction coefficient is corrected by a preset integral amount in a direction opposite to that of the deviation. Further, when the monitored air-fuel ratio is inverted from rich to lean or from lean to rich across the stoichiometric air-fuel ratio, the air-fuel ratio correction coefficient is corrected in a skipped manner by a skip amount which is set to a value greater than the integral amount, in a direction opposite to that of the deviation, so as to converge the actual air-fuel ratio to the stoichiometric air-fuel ratio. Moreover, when the output voltage of the downstream O.sub.2 sensor largely fluctuates beyond a preset rich side limit value or a preset lean side limit value, the skip amount is increased so as to largely correct the air-fuel ratio correction coefficient for completing the correction of the air-fuel ratio as quickly as possible.
Japanese First (unexamined) Patent Publication No. 3-185244 or U.S. Pat. No. 5,090,199 which is equivalent thereto, discloses another air-fuel ratio control system for an internal combustion engine.
In the system of this publication, an air-fuel ratio sensor is arranged upstream of a catalytic converter, and an O.sub.2 sensor is arranged downstream of the catalytic converter. When it is determined based on the output voltage of the O.sub.2 sensor that an air-fuel ratio of the exhaust gas is deviated to a rich or lean side with respect to the stoichiometric air-fuel ratio, the target air-fuel ratio is corrected by a preset value in a direction opposite to that of the deviation so as to converge the actual air-fuel ratio to the stoichiometric air-fuel ratio.
However, the foregoing conventional systems have the following disadvantages, respectively:
In either of the foregoing conventional systems, although the actual air-fuel ratio is controlled to be converged to near the stoichiometric air-fuel ratio as described above, no consideration is made with respect to an adsorbing condition of the harmful components in the exhaust gas to the catalytic converter. Specifically, as is known, as components in the exhaust gas, when the air-fuel ratio is deviated to the lean side, nitrogen oxide NOx and oxygen O.sub.2 are increased, on the other hand, when the air-fuel ratio is deviated to the rich side, carbon monoxide CO and hydrocarbon HC are increased. When the deviation of the air-fuel ratio is not so large, these harmful components are adsorbed in the catalytic converter so as to be prevented from emitting into the atmosphere. In the foregoing correction in the conventional systems, even when the air-fuel ratio is converged to the stoichiometric air-fuel ratio, it is possible that the harmful components remain in the catalytic converter to some extent. However, when the harmful components remain in the catalytic converter, the adsorption capability of the catalytic converter for the harmful components is decreased correspondingly, that is, tolerance against the deviation of the air-fuel ratio is decreased. As a result, when, for example, a vehicle repeats acceleration and deceleration to frequently deviate the air-fuel ratio, the harmful components remaining in the catalytic converter gradually increase so that the purification of the exhaust gas becomes insufficient thereby allowing emission of the harmful components into the atmosphere.